Thin films of copper indium gallium diselenide and/or disulfide and copper indium diselenide and/or disulfide (called CIGS and CIS) are deposited on a substrate in order to produce photovoltaic cells. Such compounds, of general formula CuGAxIn1-xSe2-ySy (where x is between 0 and 1 and y is between 0 and 2), denoted by CIGSSe, are considered as very promising and could constitute the next generation of thin-film solar cells. CIGSSe semiconductor materials have a wide direct bandgap that may be set between 1.0 and 2.5 eV, thereby allowing optimum solar radiation absorption within the solar radiation range. Record conversion efficiencies of 19.5% have been recently obtained on small-area cells. The CIGSSe compounds are also called I-III-VI2 compounds, with reference to the chemical nature of their constituents, in which:
the element Cu represents an element of column I (column 1B of the Periodic Table of the Elements);
the element In and/or the element Ga represent elements of column III (column 3B of the Periodic Table of the Elements); and
the element Se and/or the element S represent an element of column VI (column 6B of the Periodic Table of the Elements).
There are therefore approximately two atoms from column VI for one atom of column I and one atom of column III in the single-phase range around the I-III-VI2 composition of the CIGS.
The CIGS films used for photovoltaic conversion must have a p-type semiconductor character and good charge transport properties. These charge transport properties are favored by good crystallinity. Thus, CIGS compounds must be at least partially crystalline in order to possess photovoltaic properties sufficient for their application in the production of solar cells. Crystalline CIGS compounds have a crystallographic structure corresponding to the chalcopyrite system or the sphalerite system, depending on the deposition temperature. A process for fabrication such semiconductors is known from the application WO 03/094246.
Chalcopyrite materials, such as for example of the Cu(In,GA)(S,Se)2 type have bandgap widths varying between 1.0 eV for CuInSe2 and 2.4 eV for CuGaS2. The solar cells having the highest efficiencies and the commercialized modules are prepared from absorbers with Ga/(Ga+In) ratios between 25 and 30%, corresponding to bandgaps of 1.12 eV. The use of solar cells based on absorbers having a wider bandgap has two advantages: firstly, they are close to the optimum value of 1.5 eV for solar spectrum absorption and secondly, for a module application, the series resistance losses are reduced for high voltages and low currents.
Starting from CuInSe2 absorbers, it is possible to increase the width of the bandgap by substituting indium and/or selenium atoms with gallium and/or sulfur atoms respectively. The current record cells, having efficiencies of 18%, are obtained by substituting about 30% of the indium atoms with gallium atoms.
It is also possible to increase the bandgap of CuInSe2 by replacing some of the selenium atoms with sulfur atoms. This process will be called hereafter “CuInSe2 sulfurization process”.
The sulfurization of metallic or binary precursors has been described. In V. Alberts and F. D. Dejene, Journal of Physics D: Appl. Phys. 35, 2021-2025, (2002) for example, the sulfurization takes place under a pressure of elemental sulfur at high temperatures, below the softening point of glass (600° C.). In K. Siemer, J. Klaer, I. Luck, J. Bruns, R Klenk and D. Braünig, Solar Energy Materials and Solar Cells, 67, 159-166, (2001), a rapid thermal process (RTP) is used to anneal the Cu—In metal precursors at 600° C. for three minutes (total annealing plan) and with rates of temperature rise of around 10° C./s. The substrate is placed in a quartz chamber and the elemental sulfur is placed beside the substrate. A vacuum is created in the chamber before annealing. The pressure during the annealing is then the saturation pressure of sulfur.
There are other sulfurization methods for obtaining thin films of semiconductors having optimum bandgaps, such as for example the one described in document U.S. Pat. No. 5,730,852. A film of precursor having the composition CuxInyGazSen (where x, y and z are between 0 and 2 and n is between 0 and 3), using a pulsed-current method. This step is followed by a step of depositing a film of Cu+Se or In+Se elements by vacuum evaporation. A final annealing step allows the homogeneity and the quality of the resulting film to be improved.
However, these methods either involve toxic substances, implying severe constraints on the processes (use of an H2S or H2Se atmosphere) or do not allow the bandgap width to be finely controlled. They also require a vacuum step.
Moreover, when a VI element is used in its solid form (for example sulfur or selenium in powder form) close to the CIGS precursor, problems of heterogeneity of this element may arise.